Valve is "not Planning to Disable Downloading Old Builds" on Steam

Update - September 28, 2021: Valve has responded to reports that Steam might be removing the option to download older game versions.

September 28 - Valve has released an announcement with PCGamer that explains the studio does not plan to eliminate the option to download older versions of game builds. "We don't actually have plans to disable downloading old builds," a spokesperson tells the site. "What we're working on is an approach to handle edge cases that involve unowned content, and helping partners more easily take down versions that need to be removed due to copyright issues. We'll have more to share on that work when it's ready for shipping." Original story is available.

Steam beta 2 comes with an update that seems to eliminate the ability to rollback updates to games or play older versions of games on Steam. Under the new version, users are no longer able to download depots for Steam games to play earlier updates of games they own - and many are unhappy about this change.

There are many games that are drastically changed between updates - there's an entire fan group dedicated to locating and archiving the various versions of Minecraft over time, for example and other games like Counter-Strike update frequently with major changes.
minecraft server list

It is possible to download older versions of Steam games using SteamDB. All you have to do is find the version you wish to play. However, the September 23 beta update for Steam has apparently stopped this practice - so once the update is live in Steam as a whole, nobody will be able to play older versions on Steam's client.

Pavel Djundik, creator of SteamDB, confirmed that it would be impossible to download older Steam game depots and play them with the latest code. This is at a minimum if Steam enforces it across all games. Pavel adds that this means SteamDB will now have to "go back to the past in which it was able to track files for what it owns" in the event that it is implemented. SteamDB includes Pavel's full, thorough explanation.

The last Steam beta brought a significant modification to downloads for games.

With this change, it is no longer possible to download older game versions, because client asks for a code (GetManifestRequestCode) that rotates every five minutes.

- Pavel Djundik (@thexpaw) September 23 2021

Many Steam tinkerers are dissatisfied with this change and would like to reverse it. Valve will decide if this will be before the live release.